Identifying whether a transistor is shorted or open requires different methods of testing depending on the type of fault suspected. To check for a shorted transistor, you can use a multimeter in diode check mode or resistance mode. In diode check mode, place the multimeter probes across the collector-base and emitter-base junctions of the transistor. If the transistor is shorted, the multimeter will typically show a very low resistance reading (close to zero ohms), indicating a direct connection between the terminals. In resistance mode, you should also see a low resistance reading across these junctions, confirming the short circuit.
On the other hand, to identify an open transistor (where the transistor is not conducting at all), use the diode check mode of the multimeter across the same junctions (collector-base and emitter-base). In this case, a healthy transistor should show a diode-like voltage drop (around 0.6 to 0.7 volts for silicon transistors) in one direction and a high resistance or open circuit in the reverse direction. If you get no reading or a very high resistance in both directions, it indicates that the transistor junctions may be open, meaning no current can flow through them.
Determining if a transistor is broken typically involves testing it with a multimeter using two primary methods: the diode check mode and the hFE (DC current gain) measurement mode. In diode check mode, you can test each junction (collector-base and emitter-base) to ensure the transistor exhibits the expected forward voltage drop (around 0.6 to 0.7 volts for silicon transistors) and a high resistance or open circuit in reverse bias. If the transistor does not show the expected diode characteristics or if both directions show a high resistance, it may be defective.
Another method is to measure the hFE (DC current gain) of the transistor using the multimeter in hFE mode (if available). This mode applies a small current to the base of the transistor and measures the ratio of collector current to base current (hFE). A healthy transistor typically has an hFE value within its specified range (which varies depending on the transistor type and model). If the hFE measurement is significantly different from the expected value or if it fluctuates widely, it may indicate a defective transistor.
To confirm conclusively that a transistor is defective, you can perform additional tests or comparisons. For instance, compare the measurements of the suspected transistor with a known good transistor of the same type and model. If there are significant differences in the readings between the two transistors, it strongly suggests that the suspected transistor is defective. Additionally, visually inspect the transistor for physical damage such as cracks, burns, or corrosion, which could indicate internal damage affecting its performance. By combining these methods—diode check, hFE measurement, comparison with a known good transistor, and visual inspection—you can effectively confirm whether a transistor is defective and needs replacement.